This invention relates to testing alloys to determine their susceptibility to stress corrosion cracking.
Many alloys are known to fail prematurely under the combined influence of tensile stress and corrosive environments, due to the formation and growth of cracks of an almost brittle nature. This phenomenon, whereby the load bearing capability of an alloy is decreased by a corrosive service environment, is known as stress corrosion cracking (hereafter S.C.C.). Though not completely understood the S.C.C. phenomenon is described more fully in Shrier, L. L., Corrosion, J. Wiley and Sons, New York (1963 ). S.C.C. occurs under a wide variety of circumstances, and is dependent upon specific combinations of the corrosive environment and the composition and microstructure of the alloy. Quality control test procedures are required to assess the susceptibility of an alloy to S.C.C. prior to fabrication of components.
Various test procedures have been devised [see Parkins et al., Br. Corros. J. 7, 154 (1972)], consisting primarily of either so-called constant load or constant total strain tests, often conducted under conditions of alternate exposure to the corrosive solution and to air. Unfortunately, the duration of such tests is typically about 30-90 days. There are at present several approaches to reducing the test duration, for example, by applying an electric potential to the sample, or by using boiling corrosive liquids. In recent years the constant rate test has been slowly gaining in popularity for investigative purposes, and appears to be the most rapid test at the present time. However, even this procedure requires a minimum of about one day.
It is an object of this invention to provide still faster yet reliable test for determining the susceptibility of an alloy to S.C.C. It is a further object of this invention to provide a test which relies upon measuring the rate of oxidation of the elements in a previously oxidized alloy which are revealed to an electrochemical environment following the application of an external load thereto.
The invention may better be appreciated in the light of certain earlier proposed theories for S.C.C. According to these theories the surface of corrosion resistant metals is covered by an inert film, usually an oxide, which protects the electrochemically active metal from the corrosive environment. If this protective film is ruptured, as occurs during plastic deformation of the metal, the exposed metal surface is attacked or even dissolved by the corrosive environment, until a new protective or "passivating" film can reform. As plastic deformation is continued this fresh film will in turn be ruptured and the metal dissolution and repassivation process repeated, thereby generating an active path for a stress corrosion crack. The details of this mechanism have not been completely resolved, particularly with regard to the nature of the material revealed when the oxide film is ruptured and the subsequent electrochemical reactions. However, since pure metals are not susceptible to S.C.C. it seems clear that the alloying elements play a role in the S.C.C. mechanism. In some systems, for example, aluminum alloys, the path of the stress corrosion crack, which is intergranular, is preordained by the distribution of the alloying elements in the microstructure. These elements are present in the grains as strengthening precipitates, but there is preferential precipitation in the grain boundaries so that a very narrow and softer precipitate-free zone (PFZ) develops immediately adjacent to the boundaries. Plastic deformation tends to be concentrated in the PFZ.